Sealants and foam-in-place gasket materials are any of a wide variety of materials that can form a seal to prevent passage of a liquid or a gas. They are widely used in all sectors of manufacturing ranging from the high tech aerospace industry to the conventional construction industry. Sealants are generally used as a barrier or a means of protection to stop infiltration of dust, dirt, moisture, oil and the like. Gaskets, on the other hand, are typically used between matched machine parts or around pipe joints and the like to contain a gas or liquid. Both sealants and gaskets are often used to provide other important functions such as a sound, vibration and fire barrier, thermal and electrical insulator, and joining material. More frequently, they are utilized to serve multiple roles to perform several of these functions. Sealants usually consist of a mixture of chemicals and can come in the form of a liquid, semi-liquid or solid. They can be applied to substrates in the same way as adhesives by using an application equipment or by hand from a cartridge. Gaskets, on the other hand, embrace a wide range of materials coming in a variety shapes and forms. They are often supplied as pre-fabricated parts, examples of which are rubber O-rings and auto weather strips. Foam-in-place materials are a special type of gasket material similar in both composition and physical forms to sealants, and are applied by using the same types of application methods. There is considerable overlap in functional property requirements between sealants and foam-in-place gasket materials, thus, it is not atypical to see one product to be used in both places. To function properly, a sealant or a form-in-place gasket must have a good adhesion to the substrate to be joined, good flexibility, low shrinkage, softness, resistance to temperature fluctuation, easy application and sag or slump resistance.
Many types of wet sealants in the form of petroleum gels are commercially available from a variety of sources for industrial and consumer applications. These wet sealants include silicones, acrylics, polysulfides, polyurethanes and silyl polyethers. They are especially suitable for manual application by using a cartridge gun and are popular on construction sites of residential, industrial and office buildings to seal exterior and interior joints such as the gaps at corners of sidings of residential houses. These types of sealants consist of either a liquid curable oligomer or a solvent polymer and usually have poor green strength. It can take days for the strength to develop fully. They serve well in construction area where high green strength is not needed. Due to their poor green strength, these products, however, are not suitable for today's industrial assembly applications where high productivity is required. Hot melt sealants and foam-in-place gaskets based on butyl, moisture curable polyurethane and styrenic block copolymers (SBC) have overcame the green strength problem and have been used for industrial assembly. Hot melts, on the other hand, have their own shortcomings. The butyl and SBC based products do not have adequate adhesion to many plastic substrates. In addition, butyl sealant usually has high viscosity and poor application characteristics. It is usually not foamable and therefore, cannot be used as foam-in-place gasket material. Although well-formulated moisture curable hot melt polyurethanes can provide both good adhesion and green strength, these type of products are notorious in their handling and application process. From product to application, care must be taken in every step of the way to exclude moisture from the package in order to avoid pre-mature curing. The potential health hazards of toxic residue isocyanate monomer is also a concern. A need, therefore, exists for a hot melt sealant and foam-in-place gasket material that has high green strength, good adhesion to various substrates, low tack, no toxicity and easy application.
The composition of the present invention, which is based on rubber together with at least one semicrystalline olefinic polymer, can adequately meet these needs and advantageously provides good adhesion, good green strength, low or no tack, no toxicity, easy application, excellent cleaner fluid resistance. The composition has particularly excellent flexibility and resiliency to provide conformity to the shape of the gap to be sealed. This character also enables the hot melt composition to withstand the thermal stresses under temperature fluctuation, thereby offering excellent seal durability in situations where wide temperature change is the norm, such as in window and door sealing application. The thermal stresses are usually encountered due to the difference in thermal expansion coefficient of different materials at the joint. The composition can be easily foamed by using a compressed gas such as nitrogen, carbon dioxide and compressed air. It is well suited for both sealant and foam-in-place gasket applications.
Sealant and foam-in-place gasket compositions based on rubber together at with least one semicrystalline olefinic polymer are not known in the prior art although adhesives containing ethylene propylene rubber (EPR), ethylene propylene diene rubber (EPDM) and/or olefinic polymers have been disclosed in several patents. The adhesives, however, are formulated to have different characteristics from sealants and generally lack the flexibility, resiliency, temperature fluctuation resistance and foamability. The majority of prior art adhesives are either solvent based or contain curing agents, and therefore, can not be considered as hot melt in a conventional sense. Due to the presence of solvent and/or curing agents, these adhesives cannot be processed with hot melt equipment. Tynan et al U.S. Pat. No. 5,798,175 disclosed adhesive compositions based a pressure sensitive adhesive (PSA) compositions based on a natural rubber or synthetic rubber and an amorphous olefin blend comprising EPR, hydrogenated polyisoprene and atactic polypropylene (APP). The adhesives were prepared by compounding the above-mentioned polymers, a tackifier and an organic solvent. These adhesives are thus solvent based, and are thus not considered to be a hot melt.
Davis et al U.S. Pat. No. 5,612,141 and European Patent Application EPO 714963A2 disclose an adhesive tape composition comprising a polymer blend containing at least one EPDM rubber and another polymer selected from the group consisting of polyisoprene, polybutadiene, EPR and mixtures thereof. The tape composition further includes at least one tackifier and a sulfur curing package. The composition has extremely high viscosity and contains curing agents, and therefore is not a conventional hot melt.
Davis et al European Patent Application 0672737A1 discloses an adhesive tape composition comprising at least one crosslinkable EPDM, one or more tackifier(s), a filler, a softener, a curing system for the rubber polymer. This composition is, again, not a conventional hot melt.
Planthottam et al U.S. Pat. No. 5,618,883 discloses a hot melt PSA composition based on tackified EPR rubber modified with a styrene-butadiene-styrene (SBS) block copolymer and/or styrene-ethylene-butylene-styrene (SEBS) block copolymer.
Tape compositions were also disclosed in U.S. Pat. Nos. 4,379,114, 4,404,056 and 5,242,727. These compositions usually contained an EPDM, or a blend of EPDM with other elastomers including butyl, halogenated butyl, polyisoprene, and styrene-butadiene rubber (SBR), a tackifier, a curing package and other optional ingredients such as plasticizer and filler and the like.
It is apparent that all the above prior art adhesive compositions are based on EPDM or EPR rubber, either alone or in combination with other types of rubbers. It is well known to those skilled in the art that all the rubbers herein noted above are amorphous polymers. The definition of amorphous polymers and their contrast to semicrystalline polymers can be found in most polymer chemistry textbooks, an example of which is Polymers, Chemistry and Physics of Modern Materials by J. M. J. Cowie, 2nd Edition, Black Academic and Professionals.
These prior art compositions rely on curing or chemical crosslinking to acquire good cohesive strength and good adhesion characteristics. When uncrosslinked, adhesives based on EPR and EPDM have poor cohesive strength and poor bond strength. The presence of curing agents, and solvents in some cases, practically make them useless as a hot melt since the curing agents will cause gelling or crosslinking of the adhesive in the application equipment, thereby causing plugging and severe damage to the machinery. Furthermore, viscosities of the prior art adhesives are usually very high and beyond the capability of conventional hot melt coater equipment.
Wang et al U.S. Pat. No. 6,143,818 discloses a hot melt adhesive composition comprising a blend of EPR or EPDM with a semicrystalline olefinic polymer. This adhesive composition, however, does not have the flexibility, resiliency, temperature fluctuation tolerance and foamability to be used as a sealant or foam-in-place gasket material.